The present invention relates generally to melter kettles that are designed and used to melt thermoplastic materials that are applied to pavements such as roadways, airport runways, parking lots, bicycle paths and other surfaces requiring pavement markings. More particularly the present invention is directed to systems and methods to regulate heat in melter kettles that have heat domes.
A variety of thermoplastic materials and compositions have been developed and used in the roadway striping industry. In order to apply such thermoplastic materials and compositions, they have to be melted and mixed. Melting, which involves both initial melting from solid stock or feed materials and maintaining the materials/compositions in a molten state for application onto roadways and other pavements, is typically conducted in melter kettles (also referred to herein as “melting kettles”) which can be heated by electrical means, or by burning combustible fuels.
Thermoplastic materials/compositions are the current products of choice for many types of marking applications. However, unlike most other types of marking materials thermoplastic materials/compositions must be melted for use. Thermoplastic materials/compositions can be applied by various methods such as spraying, extruding, and screeding. In order to be applied to pavement surfaces the thermoplastic materials/compositions need to be melted and heated to a sufficiently high temperature so as to adjust their viscosity as needed for a particular type of application process. In addition the temperature has to be controlled to avoid scorching.
The process of pavement marking using thermoplastic materials/composition is limited in time to how long it takes to melt the thermoplastic materials/compositions and heat them to a suitable application temperature. Older melting kettles were basically cylindrical tanks that were provided with combustion chambers on their bottoms.
Originally thermoplastic materials used for pavement markings were heated using a double boiler method. A heat source such as a propane burner or a diesel burner would heat high temperature heat transfer oil to about 420° F. in a vessel (oil bath) that contained the heat transfer oil. A vessel designed to hold solid or granular thermoplastic material was encased by the oil bath. The material in the “kettle” was agitated to allow for even heating as well as maintaining a homogeneous mixture of the thermoplastic components. This was a slow melting process however the equipment used at that time was rudimentary such that the slow melting times of the thermoplastic materials matched the low production rates. First applications were performed using drag boxes. Not much material was placed in a day.
The next improvement in reducing thermoplastic melting times was the elimination of the double boiler oil bath method. First propane and later diesel fired burners with the flame directed on the kettle bottom were developed. The direct flame deformed the kettle bottom causing premature kettle failure. Sacrificial baffles were placed between the flame and kettle solving this problem. About the same time improvements were being made on melting thermoplastic, materials, other improvements were being made on more efficient application methods. The development of hand carts replaced drag boxes, followed by mobilized application applicators. Truck mounted applicators were later developed. However, these early types of applicators could only be filled using gravity methods and were limited to low capacities of a single color thermoplastic material.
As methods of transferring thermoplastic materials into melting kettles were developed that did not rely solely upon gravity, applicator truck capacities also improved dramatically. The newer trucks were developed that could carry, melt and apply more than one color of thermoplastic material. Application speeds increased and daily application capabilities increased. Application output increased from 1 ton/day in the drag box era (on a good day), to 30 tons/day for crews using modern equipment.
There remained a need to further improve kettle melting efficiencies. Heat domes of different sizes were introduced to improve the melting efficiencies of thermoplastic materials in melter kettles. The goal of heat domes was to increase the surface area of the internal thermoplastic kettle walls that are in contact with the thermoplastic volume, thereby allowing increased heat transfer to the thermoplastic materials therein. As compared to melting kettles that do not have heat domes, melting kettle having heat domes significantly reduce the melting time of thermoplastic materials using identical melting procedures. The design of heat domes was not based on actual engineering data but rather based merely on trial and error.
Heat domes (also referred herein to as “heat dome chambers”) are closed top chambers that are extend upward into the bottoms of melter kettles in a central portion thereof above lower combustion chambers. The walls and top of heat domes provide addition heat transfer surface are between the combustion chamber and thermoplastic material within a melting kettle.
The present invention provides for controlling the rate of heat exchanges from the heat domes in melting kettles. In this regard according to one aspect the present invention provides for regulating the outflow of air from heat domes in melting kettles from zero to full exhaust out flow. The present invention further allows for measurement of outflow rate and temperature.